Add a comment to TargetInstrInfo about FoldImmediate

This comment documents the current behavior of the ARM implementation of this
callback, and also the soon-to-be-committed PPC version.
PPC ISEL is a select and never has side effects
Implement PPCInstrInfo::FoldImmediate

There are certain PPC instructions into which we can fold a zero immediate
operand. We can detect such cases by looking at the register class required
by the using operand (so long as it is not otherwise constrained).